1. Field of the Invention
This invention relates to high dynamic range (HDR) imaging, and in particular, it relates to removal of ghost artifact during HDR image creation.
2. Description of Related Art
High dynamic range (HDR) imaging is a technique used in image processing and digital photography to handle sources that have extremely large ranges of brightness (light intensity). For example, an outdoor scene in daylight may include blue sky and sunlit objects as well as objects in shadows; a night scene may include neon lights and brightly lit objects as well as poorly lit objects; an indoor scene may include bright windows as well as darker areas, etc. These scenes pose a challenge for imaging devices such as digital cameras; the dynamic range of the image sensor of currently available digital cameras often cannot adequately image such scenes. If the exposure level is adequate for capturing details of darker areas of the scene, the brighter areas will often be overexposed with details lost; conversely, if the exposure level is adequate for capturing details of brighter areas of the scene, the darker areas will often be underexposed with details lost.
HDR imaging techniques deal with this problem by taking multiple images of the same scene at various exposure levels, and then digitally merging the multiple images to create an HDR image that contains information from the original multiple images, so that details in both brighter and darker areas are adequately expressed in the HDR image. Methods for creating an HDR image from multiple images are generally known.
Bracketing is a technique in photography for taking multiple images of the same scene using different exposure levels or other different setting values such as focus, depth of field, etc. Some cameras can perform autobracketing, i.e., automatically changing the setting multiple times and take multiple images. Each of the multiple images is sometimes referred to as a bracket. Multiple images generated by exposure bracketing can be used to create HDR images.
During HDR image creation, ghosting artifacts can appear when object have moved, appeared or disappeared in between the shooting of the different images (brackets). For example, during the shooting of three brackets, if a person walks into the scene only in the third bracket, then the HDR image created from the three brackets will have a semi-transparent figure of the person over the scene (“ghost”).
Methods have been proposed to identify such ghost-inducing objects within the multiple images, so that the images can be processed to reduce or eliminate ghosting effects in the resulting HDR image. For example, some known techniques for ghost removal use windows to compare brackets locally. Some of these techniques are described in a review paper, A. Srikantha and D. Sidibé, Ghost Detection and Removal for High Dynamic Range Images: Recent Advances, Signal Processing: image Communications, 27(6), pp. 650-662, 2012. U.S. Pat. Appl. Pub. No. 2011/0188744 describes a method for ghost artifact removal using a normalized cross-correlation (NCC) technique. In these known methods, corresponding local regions (windows) of different images are compared, and if the contents of the windows are deemed too un-similar, the region may be deemed to involve moving subjects. Because different brackets by design have different exposures (light intensities) and therefore different pixel values for the same pixel locations, the comparison techniques should be insensitive to the overall light intensity.